In many young stroke patients the cause of stroke remains unclear. Enhanced red blood cell aggregation is considered a factor related to the pathogenesis of stroke in elderly patients, in whom enhanced red blood cell aggregation is correlated with increased fibrinogen. We determined red blood cell aggregation, fibrinogen concentration, hematocrit value and erythrocyte sedimentation rate in 18 stroke patients ≤ 50 years of age in the early phase and in 40 stroke patients ≤ 50 years in the late phase, and compared the values with those in young control persons. We also determined these variables in stroke patients ≤ 60 years of age in the early and in the late phases and in elderly controls. In young stroke patients we found an enhanced red blood cell aggregation compared with young controls (p < 0.00005), both in the early and in the late phases, whereas fibrinogen was normal. Red blood cell aggregation was significantly associated with stroke after adjusting for fibrinogen, hematocrit and erythrocyte sedimentation rate [adjusted odds ratio 16.20; 95% confidence interval (CI) 2.80–93.61]. Red blood cell aggregation was higher in elderly patients than in elderly controls (p < 0.05). In elderly patients fibrinogen was associated with stroke (crude odds ratio 12.92; 95% CI 2.54–65.82), whereas after adjusting for red blood cell aggregation, fibrinogen, hematocrit and erythrocyte sedimentation rate only erythrocyte sedimentation rate showed a significant association with stroke (odds ratio 26.37; 95% CI 1.93–359.74). In conclusion, enhanced red blood cell aggregation independently relates to stroke in young people, which may suggest that enhanced red blood cell aggregation contributes to stroke cause, whereas in elderly patients any such effect is probably related to confounding by raised